How much work you have before you is down to the state of the house itself – it varies from one job to another.
There are many power tools here to help you prepare for the exterior of your home for painting, but when it comes down to it you'll wind up using the good old manual labor to do the job. You could look here at various exterior painting projects that are done by professionals.
Everything scraping and sanding and tear will be worth it in the end. The sweat you put into preparing the house will give you a result that is worth it.
Peeling paint and mildew (mold caused by moisture) are the main things you should look for when preparing the exterior of your home.
Any loose paint must be removed. All mildew stains should be cleaned and the area under treatment to stop the problem from returning.
If you do not properly handle mold, it is guaranteed to bubble under your freshly painted walls and ruin all your hard work. If you have aluminum siding on your house all you need to do is give it a good cleaning and perhaps Sand spots really rough.
Once you have scratched the worst damage (if any), then it's time to sand the surface. But before you start anything sanding you are better served to give the walls a wash first.
You can use a pressure washer to do this, but you must be very careful because some of them are powerful enough to actually cause damage to wood surfaces. In general, using a garden hose is sufficient for the cleaning work.